1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of handling a mobility management back-off timer and related communication device.
2. Description of the Prior Art
The 3rd Generation Partnership Project (3GPP) has developed a universal mobile telecommunications system (UMTS) which adopts a wideband code division multiple access (WCDMA) as a wireless air interface. In the UMTS, a radio access network known as a universal terrestrial radio access network (UTRAN) includes multiple Node-Bs (NBs) for communicating with multiple user equipments (UEs) (i.e., mobile stations (MSs)). As a successor of a General Packet Radio Service (GPRS) system, the UMTS provides high frequency spectrum utilization, universal coverage, and high-speed multimedia data transmission which are beneficial for the UEs and the NBs of the UMTS. Please note that, the UMTS and the GPRS system can support both a packet-switch (PS) service and a circuit-switch (CS) service, while systems older than the GPRS system support only the CS service.
A long-term evolution (LTE) system supporting the 3GPP Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3GPP as a successor of the UMTS, for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved UTRAN (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple UEs, and communicating with a core network (e.g., evolved packet core (EPC) network) including a mobility management entity (MME), a serving gateway, etc., for Non Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint transmission/reception (CoMP), uplink (UL) multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
In the abovementioned systems, congestion of a network (i.e., the radio access network and/or the core network) can be controlled by using a mobility management back-off timer. That is, when a UE transmits a signaling (e.g., for initiating a procedure/session) to the network, the network replies a rejection signaling including the mobility management back-off timer if the network is in congestion. The UE can retransmit the signaling, after the mobility management back-off timer expires (i.e., stops running). Alternatively, the UE can stop the mobility management back-off timer, when the network transmits a paging signaling (e.g., for initiating a procedure/session) to the UE. That is, the mobility management back-off timer can be stopped by the network-initiated procedure.
However, when a fake network (e.g., fake eNB or fake core network) transmits paging signalings (i.e., fake signalings) to UEs, the UEs may stop mobility management back-off timers and start to perform procedures corresponding to the paging signalings. Then, the congestion of the network (i.e., the true network) is deteriorated, and the network can not operate regularly for a long time.
On the other hand, a UE is allowed to initiate an emergency session (e.g., for making an emergency call), when the mobility management back-off timer is running (i.e., not expired). During the emergency session, it is possible that the congestion of the network is mitigated or eliminated, and the network may initiate a session management procedure for an access point name (APN) with the UE. In this situation, since the mobility management back-off timer is still running, and the UE cannot respond to the session management procedure, i.e., replying one or more signalings for performing the session management procedure. Thus, the UE may loss ongoing radio bearer when changing a routing area. Alternatively, the UE cannot resume a service after the emergency session, since the mobility management back-off timer is not stopped.
Therefore, how to solve abovementioned problems is a topic to be discussed and addressed.